The present invention generally relates to directional couplers for radio frequency equipment and more particularly relates to a radio frequency directional coupler having an integral filter for reducing undesired components of a signal input to the coupler.
A well known element for radio frequency equipment is a directional coupler. This device allows a sample of a radio frequency signal, which is input at an input terminal and output at an output terminal, to be extracted from the input signal. Properly designed, the directional coupler can distinguish between a signal input at the input terminal and a signal input at the output terminal. This characteristic is of particular use in a radio frequency transmitter in which both the input signal and a signal which is reflected from a mismatched antenna can be independently monitored. One or the other or both of these signals can be utilized in a power control circuit to control the output power of the transmitter.
Another element well known in the output circuit of a transmitter is a harmonic filter, which is employed to reduce the energy coupled to an antenna at harmonic frequencies of the desired output signal. In a system which consists of a transmitter coupled to an antenna, the harmonic filter can be relatively simple lowpass filter, but in a system where the transmitter must share the same antenna with other equipment, for example a companion receiver, the harmonic filter may take on a somewhat more complex configuration. For example, a bandpass filter which passes only a relatively narrow band of frequencies at which the transmitter is designed to operate while rejecting all other frequencies has been used in critical applications such as cellular radiotelephones. In order to achieve the lowest insertion loss within the smallest practical size, frequency resonant structures such as helical or coaxial resonators have been the choice of radio equipment designers. Unfortunately, resonant structures experience a reduction in their attenuation characteristics at frequencies which are approximately odd order harmonics of the passband frequency. Such a response is known as flyback. In order to overcome the flyback response, equipment designers have placed additional filtering in series with the resonant structure bandpass filter. One example of this additional filtering may be found in U.S. Pat. No. 5,023,866.
A radio equipment designer wishing to design high performance radio equipment may elect to employ a directional coupler, a resonant structure bandpass filter and a odd order harmonic flyback filter but heretofore has been constrained to use conventionally realized individual circuit elements. Such a configuration, with individual circuit elements, can experience potentially higher failure rates and increased size and cost of equipment.